Linear associative memory-based hardware architecture for fault tolerant ASIC/FPGA work-around

ABSTRACT

A programmable logic unit (e.g., an ASIC or FPGA) having a feedforward linear associative memory (LAM) neural network checking circuit which classifies input vectors to a faulty hardware block as either good or not good and, when a new input vector is classified as not good, blocks a corresponding output vector of the faulty hardware block, enables a software work-around for the new input vector, and accepts the software work-around input as the output vector of the programmable logic circuit. The feedforward LAM neural network checking circuit has a weight matrix whose elements are based on a set of known bad input vectors for said faulty hardware block. The feedforward LAM neural network checking circuit may update the weight matrix online using one or more additional bad input vectors. A discrete Hopfield algorithm is used to calculate the weight matrix W. The feedforward LAM neural network checking circuit calculates an output vector a( m ) by multiplying the weight matrix W by the new input vector b( m ), that is, a( m )=Wb( m ), adjusts elements of the output vector a( m ) by respective thresholds, and processes the elements using a plurality of non-linear units to provide an output of 1 when a given adjusted element is positive, and provide an output of 0 when a given adjusted element is not positive. If a vector constructed of the outputs of these non-linear units matches with an entry in a content-addressable memory (CAM) storing the set of known bad vectors (a CAM hit), then the new input vector is classified as not good.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to electronic circuits,specifically, programmable logic units such as application-specificintegrated circuits (ASICs) and field-programmable gate arrays (FPGAs),and more particularly to an improved method of overcoming design flawsin such programmable logic circuits, using software work-arounds inplace of the defective portions of the circuits, but only for thoseinput vectors affected by the fault.

2. Description of the Related Art

Modern electronic circuits use many different types of logic componentsor processing units to carry out numerous functions. In the early yearsof the semiconducting industry, these components were integratedcircuits (ICs) with narrowly limited functionalities. Early small-scaleintegration ICs contained a few logic gates (AND gates, OR gates, NANDgates, NOR gates, XOR gates, etc.) amounting to a few dozen transistors.These gates are combined in Boolean fashion to implement particularlogic circuits. Medium-scale integration increased the range ofintegrated logic available to counters and similar logic functions, andlarge-scale integration packed even larger logic functions, such as thefirst microprocessors, into a single chip. The current era of verylarge-scale integration (VLSI) offers complex processing and controlcapabilities with well over a million transistors on a single piece ofsilicon.

While many digital logic ICs became standard components which could beused to construct a variety of different microelectronic systems, therewas a clear desire to provide more customized or tailor-made ICs for theelectronics industry. As CMOS process technology has improved, andtransistors continue to get smaller, it has become easier for engineersto design larger ICs that may be adapted to a customer's specific needs;these types of ICs are referred to as application-specific integratedcircuits (ASICs).

An ASIC, like a conventional integrated circuit, includes an IC package(typically having a pin-grid array, or PGA), with the silicon chip (die)mounted in a cavity under a sealed lid. The physical size of a silicondie varies from a few millimeters on a side to over one inch on a side,but the size of an ASIC is often measured instead by the number of logicgates or transistors that the IC contains. Very large ASICs typicallyhave hundreds of thousands of gates.

ASICs are used for a wide variety of applications, such as children'selectronic toys, telecommunications, and data processing. ASICs are alsoknown as customer-specific integrated circuits (CSICs). Some ASICs whichare specific to a particular application, but are sold to many differentsystem vendors similar to a standard component, are commonly referred toas application-specific standard part (ASSPs). ASICs can be built usingcustomized circuits at every level, which is the most time-consuming andcomplicated approach, or be built using gate arrays, standard cells, orprogrammable logic devices. In a fully-customized ASIC, the designeruses no pretested and precharacterized cells for the design, and alllayers of the chip (including the “mask” layers) are designed to uniquespecifications. In programmable ASICs, all of the logic cells arepredesigned and none of the mask layers are customized.

These programmable ASICs include a plurality of logic elements andassociated interconnect resources that are easily programmed by theend-user to implement user-defined logic operations. They can beprogrammed using a personal computer (PC) or similar workstation, withappropriate software and a device programmer. Therefore, unlikefully-customized ASICs that require a protracted layout procedure and anexpensive fabrication process to implement a user's logic operation, aprogrammable ASIC may be utilized to implement the logic operation in arelatively quick and inexpensive manner.

There are generally two types of programmable ASICs: the programmablelogic device (PLD), and the field-programmable gate array (FPGA). Gatearrays are made up of base cells that contain a varying number oftransistors and resistors according to the vendor's specifications.Using a cell library (of gates, registers, etc.) and a macro library(for more complex functions), the customer designs the chip, and thevendor's software generates the masks that connect the transistors. AnFPGA is usually larger and more complex than a PLD.

A PLD may also be programmed at the customer's site. PLDs use fusiblelinks which are blown in order to open a circuit line, or “antifuse”technology which allows the circuit lines to be fused together. PLDtechniques include programmable logic arrays (PLAs) and programmablearray logic (PAL), which provide different configurations of AND and ORgates. Unlike gate arrays, which require the final masking fabricationprocess, PLDs are easily programmable in the field. Although memorydevices generally are not considered a PLD, since they generally have nologic functions, some programmable storage chips such as programmableread-only memory (PROM) may be considered a PLD if it contains programcode rather than just data.

It is very common in the ASIC/FPGA design process that only apseudo-comprehensive test is performed for a given hardware design. Thepseudo-comprehensive test basically uses a set of test vectors thatprovide the best coverage for the functional testing of the circuitunder test. Normally, to test an n-bit input circuit, 2n test vectorcombinations are required; however, if the n bits are divided into twogroups of m bits and (n-m) bits, then it requires only 2n-m+2m<2n testvectors to test all the functionality. This scheme, however, cannotguarantee 100% coverage. In reality, when the ASIC is in the field, itmay encounter some new inputs that are never tested in the factory. Thenew inputs may or may not cause the circuit to malfunction depending onhow well the circuit was designed.

If a design flaw exists, there are generally two approaches tocorrecting the problem, hardware or software. A common hardware approachto providing fault tolerance in FPGAs is to relocate the logic portionassigned to a defective configurable logic block (CLB) into a spare orother unused CLB, and to reroute signals from the defective CLB to thespare/unused CLB. One problem with this approach is that a usertypically does not know which CLB of a particular FPGA will bedefective, and usually generates configuration data intended for adefect-free device. If the user then attempts to program an FPGAcontaining a defective CLB, the programmed FPGA will not perform asintended. Therefore, the user must either discard the FPGA, or the usermust repeat the place-and-route process with a modified devicedescription indicating the location of the defective CLB. Because eachtarget FPGA may have a defective CLB in a different location, thisapproach potentially requires different configuration data for everydevice implementing the user's logic operation. This approach puts aheavy burden on the user who must potentially repeat the place-and-routeoperation for each FPGA and supply many PROM configurations.

Other hardware approaches also implement redundant circuitry in theFPGA, which is generally undesirable as it requires blocks of thecircuit to be reserved and possibly go unused throughout the life of theFPGA. Since it is not easy to fix an ASIC/FPGA at the hardware level,software work-arounds are called for as an alternative until the newchip is re-designed. When the hardware fails to perform itsfunctionality, software can be used to perform the same task, and thissoftware is referred to as a software work-around. The softwarework-arounds can be keyed to a static table of faulty vectors.

When a software work-around is employed to account for a new bug, thesystem performance usually deteriorates at least three orders inmagnitude (10³) because of the gap between hardware and software speeds.The software work-around can never provide the same level of performanceas the initial architecture embedded in the ASIC. At the system softwarelevel, instructions are typically executed on a scale of microseconds.At the hardware (sub-micron) level, instructions are performed on ascale of nanoseconds. This difference can bottleneck instructionthroughput and have an immense (negative) impact on the overallperformance of the ASIC.

This performance latency is exacerbated by the fact that, oftentimes, itis really not necessary to execute the software work-around. ASICs tendto use regularly-shaped, standard cells or blocks to simply constructionand design. When a hardware block is faulty, the fault sometimes affectsonly a small range of input vectors (i.e., input data) of the block.Nevertheless, if the block is faulty, then all vectors to that blockwill be rerouted for software processing. The system will accordinglyhave to wait on the software work-around for all input vectors, eventhough some of those vectors may not be bad. It would, therefore, bedesirable to devise an improved method of utilizing softwarework-arounds in an ASIC or FPGA, which would ensure appropriate handlingof bad vectors while still retaining the benefit of the speed of theoriginal hardware design for at least some input vectors of the faultyblock. It would be further advantageous if the method allowed new faultyvectors to be dynamically discovered and the software work-aroundmechanism appropriately updated.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved programmable logic unit, such as an application-specificintegrated circuit (ASIC) or field-programmable gate array (FPGA).

It is another object of the present invention to provide such aprogrammable logic circuit which utilizes an efficient softwarework-around mechanism for faulty hardware blocks.

It is yet another object of the present invention to provide afault-tolerant ASIC/FPGA which can identify potentially bad inputvectors from a faulty block and handle those bad vectors using asoftware work-around, while still using the faulty block for goodvectors.

The foregoing objects are achieved in a programmable logic unitgenerally comprising a plurality of interconnected hardware blocks, oneof them being faulty, a feedforward linear associative memory (LAM)neural network checking circuit which classifies input vectors to thefaulty hardware block as either good or not good, and selection meanswhich is responsive to the feedforward LAM neural network checkingcircuit classifying a new input vector as not good, for blocking acorresponding output vector of the faulty hardware block, enabling asoftware work-around for the new input vector, and accepting an outputvector from a software work-around input as the output vector of theprogrammable logic circuit. In an illustrative embodiment, the selectionmeans includes a software work-around enable signal output of thefeedforward LAM neural network checking circuit, and a multiplexerhaving two inputs respectively receiving the output vector from saidfaulty hardware block and the output vector from said softwarework-around, and a select line which is connected to the softwarework-around enable signal output of said feedforward LAM neural networkchecking circuit.

The feedforward LAM neural network checking circuit has a weight matrixwhose elements are based on a set of known bad input vectors for saidfaulty hardware block. The feedforward LAM neural network checkingcircuit may update the weight matrix online using one or more additionalbad input vectors. A feedforward LAM algorithm is used to calculate theweight matrix W (an N row by K column matrix, having elements w_(ij))according to the equation$w_{ij} = {\sum\limits_{m = 1}^{M}\quad\left( {{2z_{i}^{(m)}} - 1} \right)^{2}}$where z_(i) ^((m)) is the set of known bad binary vectors (i.e., only a0 or 1), M is the number of bad input vectors in the set of known badinput vectors, i is the row locator representing the particular badvector, and j is the column locator representing the bit location. Thefeedforward LAM neural network checking circuit calculates an outputvector a^((m)) by multiplying the weight matrix W by the new inputvector b^((m)), that is, a^((m))=Wb^((m)), adjusts elements of theoutput vector a^((m)) by its respective thresholds θ_(i) according tothe equation${\theta\quad i} = {{- \frac{1}{2}}{\sum\limits_{j = 1}^{K}\quad w_{ij}}}$where K is the total number of bits in a vector, and processes theelements using a plurality of non-linear units to provide an output of 1when a given adjusted element is positive, and provide an output of 0when a given adjusted element is not positive. If a vector constructedof the outputs of these non-linear units matches with an entry in acontent-addressable memory (CAM) storing the set of known bad vectors (aCAM hit), then the new input vector is classified as not good. Thefeedforward LAM neural network checking circuit classifies input vectorsto the faulty hardware block as either good or not good prior to thefaulty hardware generating its output vector corresponding to the newinput vector.

The above as well as additional objectives, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a diagrammatic representation of an artificial neural network(ANN) with a feedforward linear associative memory (LAM) modelconstructed in accordance with the present invention;

FIG. 2 is a high-level block diagram of one embodiment of a hardwaremodel for realizing the ANN feedforward LAM in accordance with thepresent invention;

FIG. 3 is a high-level schematic diagram of one embodiment of an offlinetraining circuit for use with the LAM checking circuit of the presentinvention;

FIG. 4 is a high-level schematic diagram of one embodiment of an onlinetraining circuit for use with the LAM checking circuit of the presentinvention; and

FIG. 5 is a high-level schematic diagram of one embodiment of a circuitfor recalculating a weight matrix used by the online training circuit ofFIG. 4.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention is directed to a method of providingfault-tolerance in a programmable logic circuit, such as anapplication-specific integrated circuit (ASIC) or field-programmablegate array (FPGA), by utilizing an efficient software work-around forfaulty hardware in the programmable logic circuit. The hardwarearchitecture envisioned by the invention allows for certain inputvectors to be handled by the original hardware design while only faultycases trigger the software work-around. In the preferred embodiment,this architecture is derived from a feedforward linear auto-associativememory neural network as discussed further below.

A generalized artificial neural network (ANN) with a feedforward linearassociative memory (LAM) model has one layer of input neurons and onelayer of output neurons, with a feedforward weight connection. Abackground discussion of this type of ANN can be found in the book“Digital Neural Networks,” by S. Y. Kung (1993). Feedforward models areone of two types of fixed-weight associative memory systems (the otherbeing the feedback type). The feedforward type may use linear andnon-linear associative memory. The feedforward type may also use theHopfield network where feedback connections are adapted to facilitaterecurrent operations. Conventional feedforward LAM models are designedto retrieve patterns in one shot. In contrast, a feedback network doesnot yield the correct pattern in a single forward pass. It isparticularly advantageous to retrieve a pattern in a one-shotcalculation for the present invention, because there is a severerestriction on the amount of time that is feasible for the algorithm todecide whether the original hardware result should be used or whetherthe software word-around should be used. In the present invention, anANN discrete Hopfield network algorithm is used to design checkingcircuitry which may be trained to identify potentially bad vectors inthe programmable logic circuit.

The integrity of an n-bit input vector is identified as “good” or “bad”based on the n-bit patterns stored in the network (auto-association). Asshown in FIG. 1, the LAM model 10 is derived from a set of input/outputpattern pairs {b^((m)), a^((m))}, that is, b^((m)) represents the inputvector, and a^((m)) represents the output vector. In this high-leveldiagram, weight matrix 12 represents the feedforward synaptic weights.The auto-association application of this model has the size of the inputvector b^((m)) equal to the size of the output vector a^((m)).

The input vectors (which may or may not be faulty) can be expressed as:b ^((m)) =[b ₁ ^((m)) ,b ₂ ^((m)) , . . . , b _(k) ^((m))]^(T)where [ . . . ]^(T) is the transpose of the matrix or vector, and k isthe number of bits in a vector (for example, for a 4-bit system, if sets0100 and 0110 are faulty, then b⁰={0,1,0,0}, and b¹={0,1,1,0}).Similarly, the output containing only faulty vectors may be expressedas:a ^((m)) =[a ₁ ^((m)) ,a ₂ ^((m)) , . . . ,a _(k) ^((m))]^(T).The feedforward topology has the input cells b^((m)) located at theinput layer and the output vectors a^((m)) at the output layer.

The main application of LAM circuit 10 is to identify (i.e., retrieve)the output information based on either full or partial information ofthe input. In other words, after the ASIC/FPGA design engineer alreadyknows some of the bad vectors in set a^((m)), then these bad vectors canbe used to train the LAM checking circuitry to classify other new inputsinto two classes: the first class contains all good vectors that willnot trigger the hardware fault, the second class contains all other bad(definitely faulty) or suspicious (potentially faulty) vectors that willor may cause the hardware to malfunction. The new input vectors are thusa set of test vector b^((m)) in which b_(n) ^((m)) ε b^((m)) may beidentical or partially identical to the set a^((m)) that was used toconstruct weight matrix 12.

For digital electronics applications, the weighting can be built from aknown set of bad vectors. Each element w_(ij) of the weight matrix W iscalculated according to the equation:$w_{ij} = {\sum\limits_{m = 1}^{M}\quad{\left( {{2a_{i}^{(m)}} - 1} \right)\left( {{2b_{j}^{(m)}} - 1} \right)}}$where M is the number of bad vectors in the set of known bad vectors,and a_(i)=b_(j). The identification (i.e., retrieve) process calculatesthe output vector a^((m)) by multiplying the weight matrix W by theinput vector b^((m)), that is:a ^((m)) =Wb ^((m)).To produce the binary-value (0 or 1) output, the elements of the vectorWb^((m)) are first adjusted by their respective thresholds θ_(i):${\theta\quad i} = {{- \frac{1}{2}}{\sum\limits_{j = 1}^{K}\quad w_{ij}}}$This procedure is carried out by threshold adjustment circuit 14.

To complete the calculation, each element is processed by a non-linearunit 16. If the adjusted value is positive, then the output ofnon-linear unit will be 1; otherwise, it will be 0.

With reference now to FIG. 2, there is depicted a hardware model 18 forrealizing the ANN feedforward LAM algorithm in accordance with thepresent invention. In this embodiment, the input vector is received bythe original hardware block 20 of the programmable logic unit 18, and isalso received by the LAM checking circuit 22. Initially it is assumedthat the input vector is a good vector and will therefore be handled bythe original hardware but, at the same time, feedforward LAM checkingcircuit 22 attempts to identify whether this current input vectorcontains bad data. If a bad vector is identified in the allowed time,then LAM checking circuitry 22 outputs a control signal which blocks theoutput from the original hardware 20 using a multiplexer 24, and signalsthe system to proceed with the software work-around and accept only thework-around output. Multiplexer 24 could be symbolic in this context,because the output of the software work-around may never physically havea path into the hardware.

The output of multiplexer 24 is determined by the select signal from LAMchecking circuitry 22. If LAM checking circuit 22 cannot classify theinput vector to the bad-vector group, then the selection signal will beasserted “low” to use the output from original hardware 20. If thechecking circuit successfully classifies the input vector as a badvector, then the selection signal will be asserted “high” to use theoutput from the software work-around. At the same time, the selectionalso serves as an enable signal to allow the work-around to start itsprocess. However, an operating system (such as Linux, Windows, Unix,etc.) can check an external signal, such as an interrupt signal, beforespending time on the work-around and also use the same checking signalto assure that it can use its own software result.

An illustrative implementation of an offline training circuit 30 for thefeedforward LAM checking hardware is shown in FIG. 3. For offlinetraining, it is assumed that the ASIC/FPGA engineer already knows thebad vectors. Initially, the LAM checking hardware block may be designedbased on these known bad vectors only, without worrying whether more badvectors may be discovered in the future. The weight matrix W in theoffline scheme never changes. If more bad vectors are found, theengineer can redesign the circuit again. In such a case, the new weightmatrix is calculated offline to replace the old W, e.g., usingflash-based FPGA technology. The actual hardware on the chip board isnever changed but its contents (i.e., the design) can be changed byelectronically erasing the old matrix and replacing it with the newvectors.

The design of offline training circuit 30 includes a memory array 32, anaddress memory management circuit 34, an n-bit shift register 36, anadder circuit 38, a register 40, a multiplexer 42, and a contentaddressable memory (CAM) 44. Memory array 32 is used to store the weightmatrix W. Address memory management circuit 34 allows access to all rowsof weight matrix 32. This address management also controls shiftregister 36 in the process of calculating θ_(i). The number of bits n inshift register 36 corresponds to the number of columns of matrix W.Adder circuit 38 calculates the threshold θ_(i)=−½Σ w_(ij), ∀j ε [1,K].Multiplexer 42 performs the non-linear function processing, i.e., if theinput from register 40 is greater than zero, then the output ofmultiplexer 42 is b_(i) ^((m)), or else the output is 0 (the calculationof a^((m))=Wb^((m))). Register 40 is used to store the value from thecurrent calculation while allowing the next calculation to begin (whichmight result in a change in the output of multiplexer 34). There areactually n number of 2:1 single-bit multiplexers 42 to calculate allcolumns of W at the same time. CAM 44 is used to identify the stored badvectors a^((m)) with the output from multiplexer 42. A hit in CAM 44(i.e., a match between one of the entries in CAM 44 and a vectorconstructed from the outputs of the multiplexers 42) indicates that apotentially bad vector has been retrieved. This line serves as theselect signal for multiplexer 24 in FIG. 2.

The same general hardware architecture for the offline training circuitmay be upgraded to perform online training. In on-line training, the LAMchecking circuit is self-trained and reconfigures itself as more badvectors are found. The on-line training process requires dynamicconfiguration of the weight matrix W. The offline architecture is stillreusable in this portion with an addition to the front end of a specialcircuit to perform the calculation of the weight matrix W. The weightmatrix W is stored dynamically online every time a new bad vector isfound. In addition, the a^((m)) vectors are updated online in anothercontent addressable memory unit, as discussed further below.

FIG. 4 illustrates one embodiment of an online training circuit 50constructed in accordance with the present invention. Online trainingcircuit 50 again includes a memory array 52, an address memorymanagement circuit 54, an n-bit shift register 56, an adder circuit 58,a register 60, a multiplexer 62, and a CAM 64. Each of these componentsserves a function similar to the corresponding component in offlinetraining circuit 30. The bad vectors are stored in the CAM database.Every time a bad vector is found, it is added to the CAM database. SRAMor flash memory 66 can be used for this purpose. It is straight forwardto update the SRAM. Since SRAM is not a permanent space, its content isinitialized at system start-up. Flash memory is preferred. Appropriatecircuitry is used to first store the content of the flash in a temporaryplaceholder such as an SRAM swap space, then adding the new vector tothe temporary space, and finally rewrite everything back to the flashmemory. One alternative is to use flash memory to download the contentto the SRAM every time the system boots up. All the updating thereforeis recorded into the SRAM. From time to time, the updating circuitry cantransfer the new database from the SRAM to the flash space.

The weight matrix W memory updating architecture is basically the sameas that used for CAM 64. The new calculated W has to be storedtemporarily in an SRAM space, then stored to the W permanentplaceholder. Address management unit 54 may be designed to allow thesame SRAM to be used as the placeholders for both CAM 64 and W memory52. Proper memory mapping will prevent the overwriting of the data.Online updating circuit 50 includes a hardware block 68 forrecalculating the weight matrix W. From the formula given above for thematrix, it can be seen that the elements of the a^((m)) vector must bemultiplied by 2, and then 1 subtracted from this product. As shown inFIG. 5, these steps can be performed using a shift register 70 to shiftleft the content of each element by one bit. The result is thensubtracted by using a subtractor 72 a. The entire quantity (2a_(i)^((m))−1) is multiplied to the entire quantity (2b_(j) ^((m))−1) createdthe same way by subtracting 1 from 2b_(j) ^((m)) using subtractor 72 b.All elements are summed together to produce the desired W matrix usingan adder 75 and a register 76 which feeds the prior value back to adder75.

The add-in checking mechanism allows the original hardware architectureto perform its intended task at the silicon speed. When the checkingmechanism finds a bad vector, the software operating system takeseffect. For example, in the case where the original hardware isabandoned totally, the software can start to calculate immediatelywithout waiting for the response from the checking block. The checkinghardware requires time t1 to perform the shift process to calculate2b_(n) ^((m)), time t2 to calculate the threshold, time t3 to adjust thethreshold using the non-linear unit, time t4 to access the CAM to findout whether this is a bad vector, and then time t5 to respond to the MUXto either accept the original output or use the software work-around.Roughly estimating the times t1, t2, t3, t4, t5 at a worst case siliconspeed, it takes a total of about 8 ns+20 ns+8 ns+20 ns+8 ns=64 ns (theoverall time for determining that the vector is bad is less than 60 ns).This time frame is much less than that of the software speed (atmicroseconds, or thousands of nanoseconds). Some drawbacks may exist dueto possible time-outs, when the checking circuit cannot arrive at anabsolute decision (i.e., no convergence found). It is safer to use thesoftware work-around anyway, which can be resolved by employing atime-out circuit.

If the ASIC/FPGA is designed with some re-configurable hardwareredundancy then it is a straightforward matter to put the work-aroundinto practice. The ANN feedforward LAM hardware architecture discussedherein is easily implemented in the hardware redundancy block, but canalso be implemented in an add-on FPGA.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore contemplated that such modifications can bemade without departing from the spirit or scope of the present inventionas defined in the appended claims.

1. A method of checking the integrity of one or more input vectors to adigital hardware block, comprising the steps of: identifying a set ofknown bad input vectors for the digital hardware block; and trainingchecking circuitry to selectively classify future input vectors to thedigital hardware block as either good or not good, using the set ofknown bad input vectors.
 2. The method of claim 1 further comprising thestep of classifying a new input vector of the digital hardware block asnot good, using the checking circuitry.
 3. The method of claim 1 whereinsaid training step trains the checking circuitry to classify as not goodboth future input vectors which are definitely faulty and future inputsvectors which are potentially faulty.
 4. The method of claim 1 whereinsaid training step trains the checking circuitry by feeding the set ofknown bad input vectors to a feedforward linear associative memoryneural network.
 5. The method of claim 4 wherein said training stepincludes the step of creating a weight matrix using a discrete Hopfieldnetwork algorithm.
 6. The method of claim 5 wherein said creating stepincludes the step of calculating the weight matrix W according to theequation$w_{ij} = {\sum\limits_{m = 1}^{M}\quad{\left( {{2a_{i}^{(m)}} - 1} \right)\left( {{2b_{j}^{(m)}} - 1} \right)}}$where a_(i) ^((m)) is the set of known bad vectors, a_(i)=b_(j), M isthe number of bad input vectors in the set of known bad input vectors, iis a row locator representing a particular bad vector, and j is a columnlocator representing a bit location.
 7. The method of claim 6 whereinsaid classifying step includes the step of calculating an output vectora^((m)) by multiplying the weight matrix W by the new input vectorb^((m)), that is, a^((m))=Wb^((m)).
 8. The method of claim 7 whereinsaid classifying step further includes the step of adjusting elements ofthe output vector a^((m)) by its respective thresholds θ_(i) accordingto the equation${\theta\quad i} = {{- \frac{1}{2}}{\sum\limits_{j = 1}^{K}\quad w_{ij}}}$where K is the total number of bits in a vector.
 9. The method of claim8 wherein said classifying step further includes the step of processingeach of the adjusted elements by a respective one of a plurality ofnon-linear units such that, when a given adjusted element is positive,an output of the corresponding non-linear unit is 1 and, when a givenadjusted element is not positive, the output of the correspondingnon-linear unit is
 0. 10. The method of claim 2, further comprising thestep of executing a software work-around for the new input vector inresponse to said classifying step.
 11. The method of claim 1, furthercomprising the step of updating the checking circuitry online.
 12. Themethod of claim 11 wherein: the checking circuitry includes afeedforward linear associative memory neural network having a weightmatrix W; and said updating step includes the step of reconfiguring theweight matrix W using one or more additional bad input vectors.
 13. Amethod of providing fault-tolerance in a programmable logic circuit,comprising the steps of: identifying a hardware block within theprogrammable logic circuit as being faulty; providing a softwarework-around for the faulty hardware block; and training checkingcircuitry to selectively classify future input vectors to the faultyhardware block as either good or not good, using a set of known badinput vectors.
 14. The method of claim 13 further comprising the stepsof: handling a new input vector using the faulty hardware block;classifying the new input vector as not good, using the checkingcircuitry; executing a software work-around for the new input vector;and in response to said classifying step, blocking an output of thefaulty hardware block corresponding to the new input vector, andaccepting an output of the software work-around corresponding to the newinput vector.
 15. The method of claim 13 wherein said training steptrains the checking circuitry to classify as not good both future inputvectors which are definitely faulty and future inputs vectors which arepotentially faulty.
 16. The method of claim 13 wherein said trainingstep trains the checking circuitry by feeding the set of known bad inputvectors to a feedforward linear associative memory neural network havinga weight matrix W calculated according to the equation$w_{ij} = {\sum\limits_{m = 1}^{M}\quad{\left( {{2a_{i}^{(m)}} - 1} \right)\left( {{2b_{j}^{(m)}} - 1} \right)}}$where a^((m)) is the set of known bad vectors, a_(i)=b_(j), M is thenumber of bad input vectors in the set of known bad input vectors, i isa row locator representing a particular bad vector, and j is a columnlocator representing a bit location.
 17. The method of claim 16 furthercomprising the step of updating the checking circuitry online, byreconfiguring the weight matrix W using one or more additional bad inputvectors.
 18. The method of claim 16 wherein said classifying stepincludes the further steps of: calculating an output vector a^((m)) bymultiplying the weight matrix W by the new input vector b^((m)), thatis, a^((m))=Wb^((m)); adjusting elements of the output vector a^((m)) byits respective thresholds θ_(i) according to the equation${\theta\quad i} = {{- \frac{1}{2}}{\sum\limits_{j = 1}^{K}\quad w_{ij}}}$where K is the total number of bits in a vector; and processing each ofthe adjusted elements by a respective one of a plurality of non-linearunit such that, when a given adjusted element is positive, an output ofthe corresponding non-linear unit is 1 and, when a given adjustedelement is not positive, the output of the corresponding non-linear unitis
 0. 19. The method of claim 18 further comprising the step ofdetermining that a vector constructed of the outputs of the non-linearunits matches an entry in a memory array storing the set of known badvectors.
 20. A circuit for checking the integrity of one or more inputvectors to a digital hardware block, comprising: a memory arraycontaining a weight matrix having elements which are based on a set ofknown bad input vectors for the digital hardware block; and means forselectively classifying future input vectors to the digital hardwareblock as either good or not good, using the weight matrix.
 21. Thecircuit of claim 20 further comprising means for creating the weightmatrix using a feedforward linear associative memory neural network. 22.The circuit of claim 20 wherein said classifying means classifies as notgood both future input vectors which are definitely faulty and futureinputs vectors which are potentially faulty.
 23. The circuit of claim 20wherein the weight matrix W is created using a discrete Hopfield networkalgorithm according to the equation$w_{ij} = {\sum\limits_{m = 1}^{M}\quad{\left( {{2a_{i}^{(m)}} - 1} \right)\left( {{2b_{j}^{(m)}} - 1} \right)}}$where a^((m)) is the set of known bad vectors, a_(i)=b_(j), M is thenumber of bad input vectors in the set of known bad input vectors, i isa row locator representing a particular bad vector, and j is a columnlocator representing a bit location.
 24. The circuit of claim 23 whereinsaid classifying means includes means for calculating an output vectora^((m)) by multiplying the weight matrix W by the new input vectorb^((m)), that is, a^((m))=Wb^((m)).
 25. The circuit of claim 24 whereinsaid classifying means further includes means for adjusting elements ofthe output vector a^((m)) by its respective thresholds θ_(i) accordingto the equation${\theta\quad i} = {{- \frac{1}{2}}{\sum\limits_{j = 1}^{K}\quad w_{ij}}}$where K is the total number of bits in a vector.
 26. The circuit ofclaim 25 wherein said classifying means includes a plurality ofnon-linear units which respectively processes the adjusted elements suchthat, when a given adjusted element is positive, an output of thecorresponding non-linear unit is 1 and, when a given adjusted element isnot positive, the output of the corresponding non-linear unit is
 0. 27.The circuit of claim 20 wherein said classifying means classifies agiven input vector as good or not good in less than 60 ns.
 28. Thecircuit of claim 20 further comprising means for blocking an output ofthe digital hardware block corresponding to a new input vector, andaccepting an output of a software work-around corresponding to the newinput vector, in response to said classifying means classifying the newinput vector as not good.
 29. The circuit of claim 20 further comprisingmeans for updating the weight matrix online using one or more additionalbad input vectors.
 30. The circuit of claim 27 further comprising: anSRAM array; a content-addressable memory for storing the known bad inputvectors; and address management means for using said SRAM to update bothsaid content-addressable memory and said weight matrix.
 31. The circuitof claim 20 wherein said classifying means classifies as not good bothfuture input vectors which are definitely faulty and future inputsvectors which are potentially faulty.
 32. The circuit of claim 20wherein said classifying means includes a content-addressable memory forstoring the known bad input vectors. 33-42. (canceled)
 43. A circuitcomprising: a logic circuit block; a memory including a weight matrixhaving elements based on a set of known bad input vectors for the logiccircuit block; and a training circuit coupled to the memory andconfigured to classifying future input vectors to the logic circuitblock as either good or not good, using the weight matrix.
 44. Thecircuit of claim 43 further comprising: a second logic circuit blockconfigured to execute software for producing an output vector inresponse to an indication from the training circuit.
 45. The circuit ofclaim 44 further comprising: a multiplexer coupled to the logic circuitblock and the second logic circuit block, wherein the multiplexer isconfigured to select between an output signal of the logic block circuitand the output vector in response to the indication from the trainingcircuit.
 46. The circuit of claim 43 wherein the training circuitimplements at least one of an artificial neural network and afeedforward linear associative memory neural network.